Device for the measurement and monitoring of a process parameter

ABSTRACT

The invention concerns a device for measuring and/or monitoring a process parameter, with a sensor, an intermittently working measurement circuit, which has at least one energy storer unit, wherein the measurement circuit or individual components of the measurement circuit are activated for a predetermined time span, the so-called active phase, and with a control center, wherein the measurement circuit and the control center are connected with one another over a two-wire line and wherein a control-/evaluation-unit is provided, which activates the measurement circuit at the earliest, when the energy in the energy storer unit has reached a predetermined level.

TECHNICAL FIELD

The invention relates to a device for measuring and/or monitoring aprocess parameter. The device includes the following components: asensor; an intermittently working, measuring circuit, which has at leastone energy storage unit, wherein the measuring circuit, or individualcomponents of the measurement circuit, each are activated for apredetermined time span, the so-called active phase; and a controlcenter. The measurement circuit and the control center are connectedwith one another over a two-wire line. According to a preferredembodiment of the device, both the energy supply to the device and thedata exchange between the measurement circuit and the control centeroccur over the two-wire line.

BACKGROUND

Because of this double-function of the two-wire line and the associatedcost saving, two-wire measurement apparatuses are being applied to anincreasing extent in industrial process technology. An importantindustry standard in this connection is the ISA-A 50.1-Standard, inwhich direct current values between 4 mA and 20 mA characterize theparticular measurement value and are transmitted over the two-wire line.

Not quite problem free in the case of two-wire measurement apparatusesis that even in the case of a very small current level of e.g. 4 mAthere still must be enough power made available over the two-wire lineto operate the measurement circuit, or its individual components. Thepower supply issue here is naturally even more critical, the higher thepower requirement of the measurement circuit, or its components,becomes.

Basically, there are two possibilities for handling the problem. Eitherthe measurement circuit is constructed of components with acorrespondingly small energy consumption—a solution, which enables acontinuous operation of the measurement circuit, or components with arelatively high energy consumption are operated intermittently. In thecase of intermittent operation, energy is consumed only during theso-called active phase, while the recovery phase following thereafter isused for charging an energy storer, which then can supply the activecomponents of the measurement circuit with the required power again inthe next active phase. As an example of a two-wire measurement apparatusof the first named kind, U.S. Pat. No. 5,672,975 is noted. As an examplefor a device with intermittent operation, European Patent EP 0 687 375B1 is noted named.

In the European patent, the measurement frequency of the measurementvalue transmitter is so designed, that the corresponding power demand isgreater than the power available over the two-wire line in the case ofminimum current and minimum voltage. Since the consumed power exceedsthe available power during the operation of the measurement valuetransmitter, a deficit appears inevitably in the power balance. As soonas a sensing circuit recognizes a deficit, the measurement circuit stopsoperation of the measurement program, until the deficit no longerexists. In short, in this known solution, a deficit is diagnosed in anenergy storer. On the basis of this deficit, a longer cycle time ispredicted to be necessary—the measurement frequency is thencorrespondingly changed. In the end, this means that it is alwaysestimated, when the energy storer will be completely charged, or chargedto a certain level. Following expiration of this estimated time, ameasurement signal is then issued. This known solution has thedisadvantage, that it accepts excessively long inactive phases.Consequently, the measurement rate of the measurement system—and thusthe measurement accuracy of the fill level measurement apparatus—islowered.

SUMMARY OF THE INVENTION

The object of the invention is to provide a device for optimizing powercontrol in a fill level measurement apparatus.

The object is achieved by providing a control/evaluation-unit, whichactivates the measurement circuit at the earliest when the energy in theenergy storer reaches a predetermined level. For this, the level can,for example, be sized such that it covers at least the energyrequirement of the measurement circuit during the active phase.Preferably, the level is, however, defined by the condition that theenergy storage unit is completely, or almost completely, charged.

Compared with the state of the art, the solution of the inventionexhibits three decisive advantages:

The device of the invention excels on the basis of an increasedreliability: While the measurement circuit of the prior art first reactswhen a critical voltage is exceeded, in the case of the solution of theinvention, the energy storer is always as completely charged aspossible. This is generally above all important, when DC/DC convertersare used between the energy storer and the energy consumer: In order todraw the necessary energy from the energy storer, the DC/DC converteraccording to I=U/R will take the current in inverse proportion to thestorer voltage from the energy storer. This means in the case of theknown solution that, for critically low storer voltages, the constantcurrent consumption from the energy storer increases, so that a recoveryof the system with decreasing voltage becomes always more improbable.

The device of the invention exhibits shorter cycle rates than the stateof the art: Since, in the case of an insufficient energy supply, theenergy is held at a level near the maximum and not at a low, criticallevel, the voltage across the current regulator, which sets the current,also decreases significantly. This residual voltage leads, moreover, asa matter of course, to a conversion of electrical energy into heataccording to the formula P=U·I. If the energy storer is always chargedcompletely, or almost completely, this voltage becomes lower and,consequently, also the energy transformed into heat. The energy fractiongained hereby consequently additionally becomes available to thecomponents of the measurement circuit.

The device of the invention is parameter independent: Since the cycletime is not calculated, but, instead, the charge state of the energystorer is watched, the device of the invention is independent of theconstant current consumption, the pulse current consumption, and theenergy supply. It only must be guaranteed that the energy storer ischarged without pulse, which means that the current of the consumer mustbe smaller than the current supply, and the energy extraction during theactive phase must not drain the energy storer too far down. This is,however, in the end a question of the sizing of the measurement circuit.

According to a preferred further development of the device of theinvention, it is provided that the process parameter is the fill levelof a fill material in a container. In particular, the sensor is a filllevel sensor, which emits measurement signals in the direction of thesurface of the fill material and which receives echo signals reflectedat the surface of the fill material. The measurement signals areelectromagnetic signals, e.g. microwave signals, or ultrasonic signals.In the case of microwave signals, they either propagate in free field orthey are directed into the container over a conductive element.Corresponding measurement apparatuses are offered and distributed by theapplicant under the designations “Micropilot” or “Levelflex” or“Prosonic”.

The actual fill level measurement value is established using theso-called travel time method on the basis of the useful, or true, echosignal of the digital envelope curve, or the echo curve. Herein, theecho curve represents the amplitude values of the echo signal as afunction of the travel time of the measurement signal on the pathbetween the antenna and the surface of the fill material. Using the echocurve, the useful echo signal is determined, which represents the signalportion that is reflected at the surface of the fill material.

Devices that determine the fill level of a fill material in a containerusing the travel time of the measurement signal use the physical lawthat the travel distance equals the product of travel time andpropagation velocity. In the case of fill level measurement, the traveldistance corresponds to twice the distance between the antenna and thesurface of the fill material. The fill level then equals the differencebetween the known distance of the antenna from the floor of thecontainer and the distance of the surface of the fill material from theantenna, as determined by the travel time measurement.

If high-frequency microwave signal are used as the measurement signals,then the echo signals are usually transformed into a lower frequencyrange using a sequential scanning, or sampling, method. The intermediatefrequency signal created by the transformation is subsequentlyevaluated. A characteristic of the intermediate frequency signal is thatit has the same course as the envelope; however, it is stretchedrelative to this by a defined time expansion factor. The advantage ofthe transformation to the intermediate frequency is principally thatrelatively slow and consequently cost-favorable electronic componentscan be used for the signal registering and/or signal evaluation. Anembodiment of a method for sequential sampling of echo-signals isdescribed in German Patent, DE 31 07 444 A1.

According to a preferred embodiment of the device of the invention, theenergy storer unit is a capacitor. A favorable further development,moreover, provides that a storer limiting unit is connected in parallelwith the capacitor. An example of a storer limiting unit is a Z-diode.

In order always to be able to be certain that the power needs of thefill level measuring apparatus are covered, an advantageous embodimentof the device of the invention provides that thecontrol-/evaluation-circuit initiates the active phase, as soon as apredetermined current value and/or voltage value has been reached in themeasuring circuit or in a component of the measuring circuit.

It has been found that it is particularly advantageous, when thecontrol-/evaluation-unit initiates the active phase first at that pointin time when the predetermined current value and/or voltage value is/areconstant over a predetermined time interval. In particular, thecontrol-/evaluation-unit provides an additional minimum wait time,before it initiates the active phase.

The invention is explained in grater detail on the basis of thefollowing drawings,

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of one embodiment of the device of theinvention,

FIG. 2 is a block diagram of the device of the invention,

FIG. 3 is a flow diagram for initiating the active phase,

FIG. 4 is a block diagram of a preferred embodiment of the clockedmeasurement circuit,

FIG. 5 is a schematic drawing of the voltage transients, which are usedpreferably for establishing the clock rate of the measurement circuit,and

FIG. 6 is a drawing of different voltage curves on a storer unit that isused in the invention.

DETAILED DESCRIPTION

FIG. 1 shows a block diagram of one embodiment of the device 1 of theinvention. A fill material 11 is stored in the container 8. A fill levelmeasurement apparatus 1 serves for determining the fill level L.Apparatus or sensor 2 is mounted in an opening 10 in the lid 9 of thecontainer 8. Transmission signals, especially microwave pulses, producedin the signal production-/transmission-unit 5 are radiated from antenna17 in the direction of the surface of the fill material 11. Themeasurement signals are reflected as so-called useful, or true, echosignals at the surface of the fill material 11. These echo signals arereceived in the receiver unit 6 and, if necessary, transformed to theintermediate frequency. In particular, as already explained above, thetime expanded digital envelope curve is formed, which describes theamplitude values of the echo signals as a function of the travel timebetween antenna 17 and the surface of the fill material 11. The correctclocking of the departure of the transmission signals and reception ofthe echo signals by the transmission unit 5 and the reception unit 6proceeds over the transmission-reception separating filter 7. It isunderstood that, instead of the one antenna 17, separate transmissionand reception antennas can be used. According to the invention, theclock rate of the measurement circuit 3 is so controlled by thecontrol-/evaluation-circuit 4, that it is assured that the powerrequirement of the device 1 of the invention is completely covered overthe two-wire line 12, 13 during the active phase, while the ‘firingrate’ is nevertheless as great as possible. The two-wire line 12, 13 isconnected to a control center 14, which in turn is connected to aninput-output device 15.

FIG. 2 is a block diagram of the essential components of the device 1 ofthe invention. The energy storer unit 16 is supplied with energy throughthe energy supply unit 21. Preferably, the energy storer unit 16 ischarged with a constant current. If the storer monitoring unit 23recognizes that the predetermined level ‘PowerGood’ has been reached inthe energy storer unit 16, then the microprocessor 22 receives theinformation that it can initiate the next active phase. The level‘PowerGood’ means, preferably, that the energy storer unit 16 isapproximately completely charged. Following that, the microprocessor 22triggers the transmission unit 5, and the transmission unit 5 issues ameasurement signal. The energy storer unit 16 supplies also themicroprocessor 22 with energy.

The initiation of an active phase, or the issuing of a measurementsignal, is preferably controlled according to a program stored in themicroprocessor 22. PowerGood and MinCycleTime are predetermined. Asalready mentioned, PowerGood characterizes preferably the level definedby the energy storer unit 16 at approximately maximum charge.MinCycleTime characterizes a maximum firing rate. This maximum firingrate is not exceeded—not even when the available energy actually wouldbe sufficient for an earlier introduction of the active phase.

According to the flow diagram shown in FIG. 3, when the level‘PowerGood’ has been reached, issue of the measurement signal, orintroduction of the next active phase, is delayed, until MinCycleTimehas expired.

FIG. 4 is a block diagram of a preferred energy supply of themeasurement circuit with current modulation. The energy storer unit 16is, in the illustrated case, a capacitor 18 with the capacity C_(s). Thecapacitor 18 is charged with a constant current is until the voltageU_(s) reaches the maximum voltage of the Zener-diode 19 or, in general,the storer limiting unit. The diode 19 is selected such that theavailable energy has optimum characteristics for the components of themeasurement circuit 3 to be driven thereby. As soon as the voltage U_(s)has reached the maximum possible voltage D_(z) on diode 19, the currentI_(s) stops flowing into the capacitor 18, but, instead, is turned intoheat by the Z-diode 19. As soon as this condition, and only when thiscondition, is completely reached or, theoretically, almost reached, theactive phase is initiated by the control-/evaluation-unit 4.

FIG. 5 shows schematically the voltage transients that are preferablyreferenced for establishing the clock rate of the measurement circuit 3.In particular, the typical course of the voltage U_(S) (see also FIG. 4)versus time t is displayed in FIG. 5. The course of the voltage iswatched, for example, with a microprocessor, which is e.g. part of thecontrol-evaluation-unit 4. During the active phase, thus during thepulse duration T_(P), a measurement signal is issued in the directiononto the fill material 11; the capacitor 18 is partially discharged.During the recovery period T_(R), the capacitor 18 is again charged.During the minimum waiting time T_(F), the capacitor is not chargedfurther. In this time, the transient of the voltage runs parallel to thetime axis. An observation of the voltage U_(S) for recognizing of thehorizontal transient during time T_(F) can, according to an advantageousembodiment of the device 1 of the invention, be referenced as acriterion for when the measurement circuit 3 initiates the next activephase.

According to an alternative embodiment of the device 1 of the invention,instead of the voltage transients, the voltage or current at the storerlimiting unit can be used as indicator for the initiating of the nextactive phase. For instance, if the current or the voltage is watched atthe diode 19, then a decision can be made as to whether the energystorer unit 16 is completely charged.

Alternatively, the voltage across the current regulator can be used: Ifthe supply voltage U_(B) is smaller than the voltage U_(S) resultingfrom the storer limiting unit (e.g. the voltage at the diode 19), thenthe above-stated criterion is never reached. In this case, the voltageacross the current sink in fully charged condition will fall below acertain threshold. With reference to the example shown in FIG. 2, thismeans that U_(B)−U_(S)=0V. Consequently, it is possible to use thevoltage across the current sink as another criterion for the next activephase. An optimum solution can be achieved by a combination of the twolast-named criteria.

A further alternative for determining initiation of the active phase isprovided by the calculation of the maximum voltage. If the lowest (afterspecification of the clamping voltage) voltage U_(S)=U_(TH) during theminimum waiting time T_(F) is calculated, then this can be used as alimit voltage for initiating the next active phase. Here, in particular,a simple comparator decides on the allowability of a new active phase.According to a preferred variant of the device 1 of the invention, it isadditionally provided that the threshold voltage U_(TH) can be afunction of the current I. This permits further optimizing of the‘firing rate’, or measurement frequency, of the fill level measurementapparatus 1. In this case, the energy storer unit 16 is no longercompletely charged, since the theoretical value for fulfilling theworst-case condition must be smaller than the actual voltage U_(S)during the minimum waiting time T_(F).

FIG. 6 shows different voltage curves at an energy storer used accordingto the invention for optimizing the firing rate of a fill levelmeasurement apparatus. An electrolytic capacitor 18 is used, forexample, as energy storer, wherein a main idea of the invention residesin holding this capacitor 18 always at a voltage level as close aspossible to the maximum available clamping voltage.

The behavior of voltage following a current consuming pulse—thus theissuing of a measurement pulse—is shown in FIG. 6 for three differentstarting levels. In this, the course of voltage of the capacitor 18 is ameasure for the energy level of the energy storer.

Preferred is that the capacitor 18 be in each case charged as close aspossible to the maximum clamping voltage U. The advantage of thissolution according to the invention is, on the one hand, to be seen inthat the loss power, which is consumed through the current regulator, isdecreased; on the other hand, the energy level of the energy storer isregulated to a higher level. This is important above all in the case ofuse of a DC—DC converter for power optimizing, since the current whichis drawn from the energy storer is inversely proportional to thevoltage. Consequently, also the recovery time is inversely proportionalto the relevant starting voltage—the higher the starting level, thesmaller the recovery time and the faster the following transmissionpulse can be issued.

Moreover, a voltage level, which approximates the maximum clampingvoltage, lies far removed from the critical point. Seen as criticalvoltage is that voltage level at which the voltage decrease as a resultof the issuing of a measurement signal is so great that the ‘point of noreturn’ is reached. When this voltage value is reached, the voltage onthe capacitor 18 falls to zero.

1. A device for measuring and/or monitoring a process parameter,comprising: a sensor; a control center; and a two-wire line whichconnects said sensor and said control center, wherein: said sensor hasan intermittently working measurement circuit, which has at least oneenergy storer unit, and a control/evaluation unit; said measurementcircuit is activated for an active phase which accounts for apredetermined time span; and said control/evaluation unit activates saidmeasurement circuit, at the earliest, when the energy in said storerunit has reached a predetermined level and, in addition, controls theclock rate of said measurement circuit, thus it is assured that thepower demand of the device is completely covered over said two-wireline, during the active phase, while the number of active phases pertime unit and the firing rate, respectively, are nevertheless as high aspossible.
 2. The device as defined in claim 1, wherein: the processparameter relates to the fill level of a fill material in a container.3. The device as defined in claim 1, wherein: said sensor comprises afill level sensor which issues measurement signals in the direction ofthe surface of a fill material in a container, and which receives echosignals reflected from the surface of the fill material.
 4. The deviceas defined in claim 1, wherein: said energy storer unit is a capacitor.5. The device as defined in claim 4, further comprising: a storerlimiting unit, wherein: said storer limiting unit is connected inparallel with said capacitor.
 6. The device as defined in claim 1,wherein: said control/evaluation unit initiates said active phase, assoon as a predetermined current voltage and/or voltage value is reachedin said measurement circuit, or a component of said measurement circuit.7. The device as defined in claim 1, wherein: said control/evaluationunit initiates said active phase, when the predetermined current valueand/or voltage value is/are constant during a predetermined timeinterval.
 8. The device as defined in claim 1, wherein: saidcontrol/evaluation unit inserts an additional minimum wait time beforeit initiates said active phase.
 9. The device as defined in claim 1,wherein: the energy supply of said device and the data exchange betweensaid measurement circuit and said control center occur over saidtwo-wire line.